The fast evolution of metal halide perovskite solar cells has opened a new chapter in the field of renewable energy. High-quality perovskite films as the active layers are essential for both high efficiency and long-term stability. Here, the perovskite films with enlarged crystal grain size and decreased defect density are fabricated by introducing the extremely low-cost and green polymer, ethyl cellulose (EC), into the perovskite layer. The addition of EC triggers hydrogen bonding interactions between EC and the perovskite, passivating the charge defect traps at the grain boundaries. The long chain of EC further acts as a scaffold for the perovskite structure, eliminating the annealing-induced lattice strain during the film fabrication process. The resulting devices with the EC additive exhibit a remarkably enhanced average power conversion efficiency from 17.11 to 19.27% and an improvement of all device parameters. The hysteresis index is found to decrease by three times from 0.081 to 0.027, which is attributed to suppressed ion migration and surface charge trapping. In addition, the defect passivation by EC significantly improves the environmental stability of the perovskite films, yielding devices that retain 80% of their initial efficiency after 30 days in ambient air at 45% relative humidity, whereas the pristine devices without EC fully degrade. This work provides a low-cost and green avenue for passivating defects that improves both the efficiency and operational stability of perovskite solar cells.

The visual aesthetic that involves color, brightness, and glossiness is of great importance for building integrated photovoltaics. Semitransparent organic solar cells (ST-OSCs) are thus considered as the most promising candidate due to their superiority in transparency and efficiency. However, the realization of high color purity with narrow bandpass transmitted light usually causes the severely suppressed transparency in ST-OSCs. Herein, we present a spectrally selective electrode (SSE) by imitating the integrating strategy of beetle cuticle for achieving narrow bandpass ST-OSCs with high efficiency and long-term stability. The proposed SSE allows for efficient light-selective passage, leading to tunable narrow bandpass transmitted light from violet to red. An optimized power conversion efficiency of 15.07% is achieved for colorful ST-OSCs, which exhibit color purity close to 100% and a peak transmittance approaching 30%. Long-term stability is also improved for ST-OSCs made with this SSE due to the light-rejecting and the moisture-blocking abilities. The realization of bright and colorful ST-OSCs also indicates the application potential of SSEs in light-emitting diodes, lasers, and photodetectors.

Flexible organic solar cells (OSCs) are very promising for use in portable power supply devices due to the advantages of low‐cost, light‐weight, and flexibility. However, the efficiencies of flexible OSCs are limited by the flexible transparent electrodes owing to their nonoptimal electrical, optical, and mechanical properties. To address these challenges, leaf‐like biomimetic electrodes are proposed to achieve an efficient light capture and glossy surface for a high‐efficiency flexible OSC. To mimic the internal anatomy of the leaf, the conformable electrode stack consists of a flexible polyimide substrate, light‐scattering polystyrene spheres, zinc oxide protecting layer, and electrically conductive silver nanowires to obtain a high transmittance, low sheet resistance, and low surface roughness. A record‐high power conversion efficiency of 16.1% is realized by a flexible OSC with the biomimetic electrode design, comparable to those rigid devices on glass. Moreover, the flexible OSC on this biomimetic electrode exhibits a robust bendability against flexural strain, retaining 85% of the initial efficiency after 5000 bending cycles at a radius of curvature as small as 1.0 mm.

in efficiency and stability with respect to material synthesis, interface modification, and device engineering. [1][2][3][4][5] Moreover, PeSCs hold the great potential as a key element essential in the applications of portable power source and wearable electronics. To bring the imagination to reality for flexible PeSCs, however, is nontrivial, and significant efforts in various aspects should be devoted in terms of low-temperature fabrication compatibility, flexible substrates, and encapsulation. An important aspect is the realization of transparent conductive electrodes (TCEs) with good optical, electrical, and mechanical properties that can replace the conventionally used indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) electrode. The ITO-or FTO-based TCEs feature superior optical transmittance (>90%) and electrical conductivity (<20 Ω sq −1 ), which benefit the realization of flexible PeSCs with a power conversion efficiency (PCE) over 18%. [6] However, these TCEs suffer from the limited mechanical flexibility because of their brittleness. [6][7][8][9][10] To address the aforementioned issue, numerous studies have been dedicated to exploring the potential candidates as a TCE for flexible optoelectronic devices, such as conductive polymer, graphene, and metallic nanostructures. [11][12][13] PCE over 17% has been achieved with the utilization of ITO-free TCEs. [14,15] In particular, metallic nanostructures are regarded as an effective TCE for flexible devices in terms of mechanical flexibility, electrical conductivity, and large-area film uniformity. However, the huge permittivity mismatch between metals and the surrounding media results in a tremendous reflection, which goes against the light transmission purpose. Many solutions have been carried out to suppress the reflection of metal electrodes, including the ultrathin metal films with a thickness less than 20 nm, the use of grid patterns with low coverage, as well as the fabrication of metal nanowire frameworks. [16][17][18][19][20][21][22][23][24] However, the undesirable bump surfaces of TCEs based on metal grids and metal nanowires may introduce additional traps or affect the surface morphology of the subsequently deposited films. [15,21,25,26] These obstacles will thus lead to the unwished charge recombination at the electrode/perovskite interfaces and high leakage current, which further degrade the device performance. Although a flat surface can be achieved for the ultrathin metal film using Flexible perovskite solar cells (PeSCs) possess the extraordinary potential in the applications of portable power sources and wearable electronics because of their superiorities in ease fabrication and efficient power conversion. These features are reinforced when a transparent conductive electrode with good flexibility is achieved. A unique electrode based on Ag periodic mesh (APM) perforated with hexagonal close-packed nanoholes is proposed, showing a square resistance of ≈19.0 Ω sq −1 and optical transmittance of 66.0% in the whole visible region.PeSCs m...